Study on migration characteristics of multi-component gases driven by the cold source of passive containment cooling system in a large enclosed space

Abstract

The passive containment heat removal system (PCS), as a key cooling source within the new-generation nuclear containment, exerts a significant influence on the migration and stratification behaviors of hydrogen-steam-air multicomponent gas under severe accident conditions, which is crucial for hydrogen risk safety analysis. However, most existing studies generally overlook the impact of the PCS, which limits the ability of their conclusions to provide reliable guidance for hydrogen safety analysis under severe accident conditions. Therefore, this study conducts numerical simulations to investigate the migration of multicomponent gases and hydrogen stratification characteristics under the combined effect of the PCS and buoyant jet within a large enclosed space, focusing on the depressurization phase of severe accident (where the heat removal power of the PCS exceeds the enthalpy of the released gas). The results indicate that the gas temperature stratification and concentration stratification induced by the PCS can transform the multicomponent gas buoyant jet into a “negative buoyancy jet” during its upward migration. This transformation significantly inhibits the migration of multicomponent gases toward the top of the large enclosed space, and hydrogen accumulates below the stratification interface when the interaction Froude number Fr_i < 1. Within the range of parameters studied in this paper, the peak hydrogen concentration is significantly higher than that in the scenario without the PCS cold source. Furthermore, increasing the installation elevation of the PCS not only enhances its heat removal power but also alleviates hydrogen accumulation. These findings provide a critical theoretical basis for clarifying severe accident phenomena and optimizing hydrogen mitigation strategies in the new-generation nuclear containment.